Fix development installation on Windows

Fixes 3107e41

Since that commit, installing benchexec[dev] would require installation
of pystemd, which does not make sense and does not work on Windows.
We can fix this by declaring pystemd as a dependency only for
non-Windows platforms.

setup.cfg

@@ -70,7 +70,7 @@ dev =
   pytest
   lxml
 systemd =
-  pystemd >= 0.7.0
+  pystemd >= 0.7.0; platform_system != 'Windows'
 
 [options.entry_points]
 console_scripts =